Method for producing SOI wafer

ABSTRACT

Hydrogen gas is ion-implanted into a wafer for active layer via an oxide film. The wafer for active layer is bonded with a supporting wafer using the oxide film as the bonding surface. The bonded wafer is subjected to a heat treatment at the temperature in a range of 400° C. to 1000° C. As a result of this heat treatment, the bonded wafer is cleaved at the site of ion-implanted layer as the interface thereby producing an SOI wafer. In this heat treatment for cleavage, the temperature difference within the surface of the bonded wafer is controlled to be within 40° C. Consequently, the wafer can be cleaved and separated completely across its entire surface at the site of the ion-implanted layer as the interface without leaving any regions uncleaved.

FIELD OF THE INVENTION

The present invention relates to a method for producing an SOI wafer,and more particularly to a method for producing an SOI wafer incombination with a cleavage method comprising a hydrogenion-implantation.

DESCRIPTION OF THE PRIOR ART

An SOI wafer has been used in an LSI of high-speed low-powerconsumption, as it is considered superior to a conventional siliconwafer in some properties, including separation between devices, reducedparasitic capacity between a device and a substrate and athree-dimensional structure to be feasible.

One of methods for producing the SOI wafer in the prior art includes thesmart cut method in which hydrogen ion is implanted into a silicon wafersurface and then the silicon wafer is bonded to another wafer, andsubsequently thus obtained bonded wafer is subjected to a heat treatmentfor cleavage so as for a part of the silicon wafer to be cleaved andseparated away at the site of ion-implanted layer serving as aninterface layer. In this smart cut method, the wafer for an active layerthat has been implanted with the hydrogen ion from the silicon wafersurface is bonded with the supporting wafer. Consequently, a bondedwafer having an insulating layer in the site of bonding interface isformed. After this step of process, thus bonded wafer is subjected to aheat treatment for cleavage. As for a set temperature used in the heattreatment for cleavage, as disclosed in Patent Document 1, for example,the heat treatment has to be carried out at approximately 500° C. orhigher.

[Patent Document 1]

-   -   Japanese Patent No. 3048201

SUMMERY OF THE INVENTION

Problem to be Solved by the Invention

However, in this method, if a temperature distribution in a furnace,especially the temperature distribution within the surface of the bondedwafer, during the heat treatment for cleavage turns to be uneven, are-arrangement of crystal and a void concentration within the surfaceare apt to be discontinuous. Further disadvantageously, there is a fearthat it tends to deteriorate the uniformity of film thickness of an SOIlayer, which would affect to subsequent steps for reducing the filmthickness. There would be another problem that the above method producesa region in which the cleavage would not occur at the site ofion-implanted layer as the interface, inhibiting the SOI wafer frombeing produced.

An object of the present invention is to provide a method for producingan SOI wafer according to the smart cut method, in which the temperaturewithin the surface of the bonded wafer is made substantially uniformduring the heat treatment for cleavage.

Another object of the present invention is to provide a method forproducing an SOI wafer allowing for a complete cleavage through theentire wafer at the site of the ion-implanted layer serving as theinterface without leaving any uncleaved region.

Means to Solve the Problem

A first invention provides a method for producing an SOI wafer,comprising the steps of:

ion-implanting of a noble gas element to a wafer for active layer via aninsulating film to form an ion-implanted layer in the active layerwafer;

subsequently bonding the active layer wafer with a supporting wafer viathe insulating film disposed therebetween to form a bonded wafer; andthen

heat treating the bonded wafer while holding it at a set temperature soas to cleave and separate a part of the active layer wafer at a site ofion-implanted layer as an interface, wherein

in the heat treatment, a temperature variation within a surface of thebonded wafer is controlled to be 40° C. or smaller so as to achieve thecleavage at the site of said ion-implanted layer as the interface.

In the method for producing the SOI wafer according to the firstinvention, firstly the ion-implanted layer is formed in the active layerwafer (e.g., a wafer comprising an oxide film formed on top of a siliconwafer). Subsequently, this active layer wafer is bonded with thesupporting wafer (e.g., a silicon wafer) via the insulating filminterposed therebetween. This bonding process of the active layer waferwith the supporting wafer consequently produces a bonded wafer with theintervention of the insulating film at the site of bonding surface.After this step of processing, the bonded wafer is heat treated at theset temperature (e.g., temperature in a range of 400° C. to 1000° C.).During this process, the temperature variation within the surface of thebonded wafer has been controlled within 40° C. Accordingly, bubbles ofnoble gas are formed in the ion-implanted layer, and the cleavage occursat the site of ion-implanted layer as the interface. In this case, sincethe temperature within the surface of the bonded wafer has been heldsubstantially uniform, it hardly produces any regions that will be leftwithout occurrence of cleavage but a complete cleavage would be inducedover the entire surface of the bonded wafer at the site of ion-implantedlayer as the interface.

Thus, according to the first invention, in the method for producing theSOI wafer in accordance with the smart cut method, the temperaturevariation within the surface of the bonded wafer during the heattreatment for cleavage is controlled within 40° C. For example, assumingthat the temperature at which the cleavage starts is 410° C. or higher,the cleavage takes effect in a region in the surface of the bonded waferwhere the temperature is in a range of 410° C. to 450° C. However, thewafer is forced to be cleaved and separated at the site of bondinginterface specifically in a region at the temperature of 400° C.Consequently, the SOI wafer partially lacking the SOI structure would beproduced. It implies that the temperature variation within the surfaceof the bonded wafer immediately after the starting of the cleavageshould be controlled to be within 40° C.

A diameter of the bonded wafer to be treated may be, for example, 200 mmor 300 mm.

Preferably, a heat treatment device used to provide the heat treatmentfor cleavage is an RTP (Rapid Thermal Processing) furnace of singlewafer processing type. This is because the bonded wafer can be heatedrapidly and uniformly by applying lamp heating to the wafers one by onein the single wafer processing type of furnace. On the other hand, it isdifficult to control the temperature variation within the surface of thebonded wafer to be within 40° C. in a furnace of batch type where theheating rate of the bonded wafer varies depending on the introductiontemperature and the introduction rate. If the batch type of furnace isused, the rate of temperature increase to the cleavage startingtemperature should be controlled to be 20° C./min, for example, forcarrying out the heat treatment for cleavage. With the rate oftemperature increase over 20° C./min, there will be a local regionwithin the wafer surface that is not able to follow the temperatureincrease. This may cause uneven temperature distribution across thewafer surface. Accordingly, if the rate of temperature increase is setto be 20° C./min or lower, the temperature variation over the entiresurface of the bonded wafer can be controlled within 40° C. easily andaccurately.

A second invention provides a method for producing an SOI wafer asdefined in the first invention, in which the set temperature is in arange of 400° C. to 1000° C.

In the method for producing the SOI wafer according to the secondembodiment, the set temperature for the heat treatment for cleavage ofthe bonded wafer has employed a range of 400° C. to 1000° C. The settemperature below 400° C. may be associated with a problem that thecleavage starting temperature of the bonded wafer would not be reached,failing in inducing the cleavage. With the temperature over 1000° C.,such a problem may occur that a reaction tube (silica tube) of the heattreatment device could be deformed. There will be another problem arisenthat the portions of the bonded wafer, which have been once separated,are brought into contact with each other at the site of interface andthus form a bond therein, where it will be difficult to make a cleavagelater again. Specifically, it is required that the entire surface of thebonded wafer should be in a temperature range of 400° C. to 1000° C. andthat the temperature variation within the surface should be 40° C. inmaximum.

For the case employing the batch type of furnace, the temperature withinthe furnace during the introduction is required to be suppressed to 400°C. or lower, and if taking the throughput in production of the SOI waferinto account, preferably the temperature within the furnace during theintroduction is in a range of 300° C. to 400° C. That is, the bondedwafer can be heated while keeping the small temperature variation withinthe surface of the bonded wafer by setting the introduction temperatureat 400° C. or lower.

A third invention provides a method for producing an SOI wafer asdefined in the second invention, in which the heat treatment isperformed while holding the bonded wafer substantially vertical so as toinduce the cleavage in the bonded wafer at the site of ion-implantedlayer as the interface.

A heat source for heating the bonded wafer while holding the bondedwafer substantially vertical is disposed in proximal to the top and theback surfaces of the bonded wafer. The heat source may employ aninfrared lamp, for example.

In the method for producing the SOI wafer according to the thirdinvention, the bonded wafer is held vertical by a supporting jig made ofsilica disposed in a reaction chamber within the RTP furnace. Then, thisbonded wafer is heat treated by the infrared lamp disposed laterallywith respect to the top and the back surfaces of the bonded wafer.

The reason why the bonded wafer is held vertical is to prevent thecleaved and separated wafers after the heat treatment for cleavage fromcoming into contact with each other again. Then, the entire surface ofthis bonded wafer is heated uniformly and rapidly to thereby make thetemperature within the surface of the bonded wafer substantiallyuniform. Thus, the complete cleavage can be achieved across the entiresurface of the bonded wafer at the site of the ion-implanted layer asthe interface.

Effect of the Invention

According to the present invention, in the method for producing the SOIwafer in accordance with the smart cut method, a bonded wafer is heattreated at a set temperature in a range of 400° C. to 1000° C. In thisprocess, a temperature variation within the surface of the bonded waferis controlled to be within 40° C. Consequently, bubbles of noble gas areformed in an ion-implanted layer, and the bonded wafer is cleaved andseparated (i.e., a part of the active layer wafer is separated from theremaining part of the SOI wafer) at the site of the ion-implanted layerspecifically in a region containing thus formed bubbles as theinterface. Under this condition, the temperature within the surface ofthe bonded wafer has been substantially uniform. Owing to this, thecomplete cleavage can be achieved across the entire surface of the waferat the site of the ion-implanted layer as the interface withoutproducing any region that have been left not-cleaved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing a configuration of a device for providinga heat treatment for cleavage of a bonded wafer according to oneembodiment of the present invention;

FIG. 2 is a side view showing a configuration of a device for providinga heat treatment for cleavage of a bonded wafer according to oneembodiment of the present invention; and

FIG. 3 is a side view showing a configuration of a transport device fortransporting a bonded wafer to a device for providing a heat treatmentfor cleavage according to one embodiment of the present invention.

DESCRIPTION OF REFERENCE NUMERALS

11 SOI wafer

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be describedwith reference to the attached drawings.

First Embodiment

One embodiment of the present invention will now be described withreference to FIGS. 1 to 3.

A method for producing an SOI wafer 11 in accordance with the smart cutmethod according to the present invention is carried out in thefollowing manner.

Firstly, two pieces of silicon wafer are prepared, which have been madeby slicing an ingot of monocrystal silicon grown in the CZ method anddoped with boron. One of those prepared silicon wafers serves as anactive layer wafer and the other of those silicon wafers serves as asupporting wafer. Then, an oxide film is formed on top of the siliconwafer prepared to be the active layer wafer. The formation of thesilicon oxide film is achieved by introducing the silicon wafer into anoxidizing furnace and heating the silicon wafer at a predeterminedtemperature for a predetermined time. The silicon oxide film to beformed in this process may have a thickness of 150 nm.

Secondly, the silicon wafer with the oxide film formed thereon is set ina vacuum chamber of an ion-implanting device. Then, a predetermined doseof hydrogen ion is implanted into the active layer wafer from itssurface through the oxide film. Consequently an ion-implanted layer isformed in the active layer wafer at the predetermined depth (extendingacross a predetermined range of depth within the silicon wafer).

Subsequently, the active layer wafer with the hydrogen ion implantedtherein is bonded with the supporting wafer at the side of ion-implantedsurface (i.e., oxide film surface) used as the bonding surface for theactive layer wafer. Resultantly, a bonded wafer including an insulatingfilm (i.e., the oxide film) intervening in the bonding surface isformed.

Those steps to be taken up to this stage of processing are analogous tothe steps employed in a method for producing the SOI wafer in a typicalsmart cut method.

A device used for the heat treatment for cleavage of the bonded wafer 11will now be described.

An RTP furnace 25 of single wafer processing type is employed as thedevice for providing the heat treatment for cleavage, as shown in FIGS.1 to 3. This RTP furnace 25 can provide the control of the rate oftemperature increase by as short as a unit of second in contrast withthe batch type of furnace, so that it can heat the surface of the bondedwafer uniformly and rapidly.

A reaction chamber 20 is arranged in the RTP furnace 25. The reactionchamber 20 is provided with a supporting jig 22 made of silica forholding the bonded wafer 11 vertical. Further, a plurality of infraredlamps 21 serving as a heat source used for the heat treatment of thebonded wafer 11 is disposed laterally with respect to the top and theback surfaces of the bonded wafer 11 held vertically. The infrared lamphas an elongated configuration, and a plurality of those lamps isdisposed vertically in the lateral sides with respect to the top and theback surfaces of the bonded wafer 11. This arrangement allows for theheat treatment for cleavage to be applied across the entire surface ofthe bonded wafer 11. Further, as shown in FIG. 3, a gas inlet port 26 isprovided for introducing a gas into the reaction chamber 20.

Subsequently, a method of heat treatment for cleavage of the bondedwafer 11 will now be described.

The bonded wafer 11 comprising the active layer wafer with theion-implanted layer formed therein is held horizontal in a wafer charier23, as shown in FIG. 3. The bonded wafer 11 is taken out of the wafercarrier 23 by using a wafer transport robot 24. Further, the bondedwafer 11 taken out of the carrier 23 is held vertical and transportedinto the reaction chamber 20 of the RTP furnace 25. The bonded wafer 11after having been transported into the reaction chamber 20 is supportedvertical by the supporting jig 22 made of silica in the reaction chamber20. In this step of process, the temperature during the introduction ofthe bonded wafer 11 into the reaction chamber 20 is a room temperature.

Then, the temperature within the reaction chamber 20 is increased fromthe room temperature to a set temperature defined by a range of 400° C.to 1000° C. (e.g., 500° C.) at a rate of temperature increase of 20°C./min, and the wafer 11 is held therein for 10 minutes. It is to benoted that if the rate of temperature increase exceeds 20° C./min, thetemperature distribution across the wafer surface and thus a filmthickness distribution across the SOI layer (i.e., the active layer)could be deteriorated. Therefore, preferably the rate of temperatureincrease should be 20° C./min or lower (see the result of below Table3).

The reaction chamber 20 is full of N₂ gas atmosphere as the N₂ gas hasbeen introduced through a gas inlet port 26 into the reaction chamber20. Under this condition, the bubbles of hydrogen ion that has beenimplanted into the active layer wafer of the bonded wafer 11 are formed.Then, at the site of the ion-implanted layer containing thus formedbubbles as the interface, a part of the active layer wafer (a part ofthe bonded wafer 11) is cleaved and separated over its entire surface.

In this stage of process, the wafer 11 held vertical is heated uniformlyand rapidly over its entire surface by the infrared lamps 21 disposedlaterally with respect to the top and the back surfaces of the bondedwafer 11. This makes the temperature within the surface of the bondedwafer substantially uniform. This induces a complete cleavage andseparation of the wafer at the site of the ion-implanted layer as theinterface. Further, the bonded wafer 11 is heat treated for cleavage asit is held to be vertical. Therefore, this can prevent the portions ofwafers after their having been cleaved and separated from being broughtinto contact with each other later again.

After the heat treatment for cleavage, a haze level in the cleavedsurface (defined in the active layer wafer side) of the bonded wafer 11was measured by using a surface detector (SFS6220). This was performedbecause the haze level tends to change in dependence on the thickness ofthe SOI layer and BOX layer (i.e., the buried oxide film) in the SOIwafer. The result from the measurement showed that the haze level equalto or higher than 2000 ppm was not confirmed in the cleaved surface ofthe bonded wafer 11, thus demonstrating that the cleavage process hadbeen successfully performed to form the uniform cleavage over the entiresurface.

The bonded wafer 11 after the cleavage is processed based on a typicalmethod for producing an SOI wafer in accordance with the smart cutmethod so as to be finally produced as an SOI wafer. Specifically, theheat treatment is applied to the bonded wafer 11 in order to firmly bondthe portion for the active layer wafer with the portion for thesupporting wafer. As for the condition for this heat treatment, itshould be performed at the temperature of 1100° C. or higher in theoxidizing gas atmosphere for approximately two hours.

Finally, the SOI layer surface is ground, and then thus ground surfaceis further mirror polished so as to reduce the film thickness of the SOIlayer for finishing the SOI wafer.

Subsequently, for the heat treatment for cleavage of the bonded wafer11, an experiment was conducted under different conditions as definedbelow. Specifically,

(1) an experiment in order to determine the SOI layer distribution andthe presence of any regions where the cleavage has not occurred byvarying the temperature difference within the surface of the bondedwafer 11 during the heat treatment for cleavage (Table 1);

(2) an experiment in order to determine the SOI layer distribution andthe presence of any regions where the cleavage has not occurred on thebonded wafer 11 that was introduced into a batch type of furnace underthe condition of temperature in a range of 300° C. to 700° C., and afterthe temperature having increased to 700° C., held for 30 minutes, andthen taken out of the furnace (Table 2); and

(3) an experiment in order to determine the SOI layer distribution andthe presence of any regions where the cleavage has not occurred on thebonded wafer 11 that was introduced into a batch type of furnace underthe condition of temperature at 300° C., and subjected to the heattreatment for cleavage with different rates of temperature increase upto 500° C., and then taken out of the furnace (Table 3), were conducted,respectively.

It is to be noted that a surface inspection apparatus (SFS6220) was usedfor evaluation (of the SOI layer distribution) of the cleaved surface ofthe bonded wafer 11 in those experiments. Results are shown in Table 1to Table 3.

TABLE 1 In-surface temperature difference 35° C. 40° C. 45° C. 50° C.60° C. SOI layer NIL NIL Present Present Present distribution Region ofno NIL NIL NIL NIL Present cleavage occurred

TABLE 2 Introduction temperature 300° C. 400° C. 500° C. 600° C. 700° C.SOI layer NIL NIL Present Present Present distribution Region of no NILNIL NIL NIL Present cleavage occurred

TABLE 3 Rate of temperature increase 10° C./ 20° C./ 5° C./min min min25° C./min 30° C./min SOI layer NIL NIL NIL Present Present distributionRegion of no NIL NIL NIL NIL Present cleavage occurred

It has been confirmed from the above experiment results that if thetemperature difference within the surface of the bonded wafer 11 iscontrolled to be within 40° C., then the bonded wafer 11 can be cleavedand separated completely across the entire surface. It has been furtherconfirmed that if the heat treatment for cleavage is carried out withthe set temperature in a range of 400° C. to 1000° C., when employingthe batch type of furnace, then the bonded wafer 11 can be cleaved andseparated completely across the entire surface without leaving anyuncleaved regions. It has been also confirmed that if the heat treatmentfor cleavage is carried out with the rate of temperature increase set to20° C./min or lower, when employing the batch type of furnace, then thebonded wafer 11 can be cleaved and separated completely across theentire surface without leaving any uncleaved regions.

1. A method for producing an SOI wafer, comprising ion-implanting anoble gas element in an active layer wafer via an insulating film toform an ion-implanted layer in the active layer wafer; subsequentlybonding the active layer wafer with a supporting wafer via theinsulating film disposed therebetween to form a bonded wafer; and thenperforming heat treatment on the bonded wafer while holding the bondedwafer at a set temperature so as to cleave and separate a part of theactive layer wafer at a site of the ion-implanted layer as an interface,and controlling the heat treatment so that temperature variation withina surface of the bonded wafer is 40° C. or less so as to achieve thecleavage at the site of the ion-implanted layer as the interface bypositioning a plurality of heat sources, laterally with respect to a topsurface and a back surface of the bonded wafer, so that the heat sourcesuniformly heat the surface of the bonded wafer.
 2. A method forproducing an SOI wafer in accordance with claim 1, in which the settemperature is in a range of 400° C. to 1000° C.
 3. A method forproducing an SOI wafer in accordance with claim 2, in which the heattreatment is performed while holding the bonded wafer substantiallyvertical so as to induce the cleavage in the bonded wafer at the site ofthe ion-implanted layer as the interface.
 4. A method for producing anSOI wafer in accordance with claim 1, in which the heat treatment isperformed while holding the bonded wafer substantially vertical so as toinduce the cleavage in the bonded wafer at the site of the ion-implantedlayer as the interface.
 5. A method according to claim 1, wherein theplurality of heat sources are a plurality of infrared lamps.